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Spatial and temporal dynamics of Australian rainforests

Abstract The loss of tropical and subtropical rainforest biodiversity due to increasing anthropogenic pressure lends urgency to understanding the processes which drive species coexistence. Without an understanding of how species are distributed across the landscape and how species assemblages change through time, we cannot derive appropriate management regimes for their persistence. This thesis examines the role of deterministic drivers of spatial and temporal dynamics of Australian rainforests and investigates the potential impacts of a changing climate on Australian rainforests. The findings from three studies are presented in four chapters which examine floristic turnover in situ and at catchment, regional and continental scales. Compositional turnover in tropical rainforest following the passage of a category three cyclone was examined both at five years’ post-cyclone recovery and in the context of local and regional spatial turnover. After five years, the forest remained in an active state of recovery with an approximate 30% increase in stems, 5% decrease in basal area and a 16% increase in species richness. Local spatial turnover suggests differential impacts of cyclones over even short distances and overall, a high degree of temporal stability in these rainforests, despite the impact of frequent catastrophic disturbances. Compositional turnover in subtropical rainforest along steep moisture and temperature gradients was investigated and described along an altitudinal transect in subtropical rainforest. The identification of significant modelled climatic and mapped soil variables suggests that moisture stress is an important driver of floristic turnover in these forests. Existing high levels of turnover across tree assemblages from low to mid elevations in subtropical rainforest were identified. Such turnover is greatly reduced at higher elevations. With increasing atmospheric temperatures, the cloud cap is expected to rise and we predict that subtropical rainforest communities which currently sit at the level of the cloud base (800-900m) will experience increasing in situ floristic turnover. Our findings agree with predictions for cloud forests elsewhere: high elevation endemic species will face an increasing risk of extinction as mesic climatic envelopes move upslope out of reach. Baseline data from this study will be used as a benchmark against which to formulate and test hypotheses for climate induced floristic and structural shift. It is also acknowledged that monitoring floristic turnover as a surrogate of shifting climatic envelopes may be confounded both by a lack of knowledge regarding the underlying turnover rates of rainforest communities and by the disparity in temporal scales of tree community turnover and accelerating anthropogenic climate change. Finally, generalized dissimilarity modelling is utilised to combine disparate biological survey data and remotely sensed environmental data to investigate the determinants of floristic turnover at the regional scale. Generalized dissimilarity modelling identified four environmental predictors of β-diversity in subtropical rainforest, all closely linked with moisture stress: radiation of the driest quarter, precipitation of the driest period, slope and aspect. Ten land classes were identified and mapped for the Mt Warning Caldera and may act as appropriate management units for future climate change planning within the region. This thesis has identified a potential threat to the biodiversity of Australian rainforests under a changing climate. Increasing levels of evapotranspiration, moisture stress and an increased return rate and intensity of disturbance are predicted to lead to the upslope movement of species ranges, increasing levels of in situ floristic turnover, and will likely result in the emergence of novel rainforest communities not present under current conditions. The potential for anthropogenic climate change to impact upon native vegetation communities has emphasised the need for the continuation and expansion of monitoring programs and the development of dynamic management regimes.